The present study examined mediators and moderators of the relation between parental ADHD symptomatology and the development of child attention-deficit/hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD) symptoms across the preschool years. Participants included 258 (138 boys) 3-year-old children (M = 44.13 months, SD = 3.39) with and without behavior problems and their parents who took part in a 3-year longitudinal study. Maternal ADHD symptoms predicted later ADHD symptoms in children, controlling for early child symptomatology. Both family history of ADHD and paternal comorbid psychopathology predicted later child ADHD and ODD symptoms, but they did not account for the association between maternal and child ADHD symptoms. Although paternal ADHD symptoms were associated with age 3 child ADHD symptoms, they did not significantly predict later child ADHD symptoms controlling for early symptomatology. Family adversity moderated the relation between maternal ADHD and child ADHD symptoms, such that the relation between maternal and child ADHD symptoms was stronger for families with less adversity. Maternal overreactive parenting mediated the relation between maternal ADHD symptoms and later child ADHD and ODD symptoms. Our findings suggest that targeting paternal comorbid psychopathology and maternal parenting holds promise for attenuating the effects of parental ADHD on children’s ADHD.